1. Technical Field
This disclosure relates to a lubricant coater for applying a lubricant on a surface of an image hearing member such as photoconductor, and also relates to an image bearing unit using the lubricant coater, and an image forming apparatus using the lubricant coater.
2. Description of the Related Art
An electrophotographic image forming apparatus generally forms an image according to the following process. Firstly, an image bearing member such as photoconductor whose surface has been uniformly charged by a charging device is subjected to laser scanning to form a latent electrostatic image, and the latent electrostatic image is developed by a developing device to form a toner image. Subsequently, a toner image obtained by the developing step is directly transferred onto a recording medium such as transfer paper from the image bearing member or transferred onto recording paper via an intermediate transfer member. A slight amount of untransferred toner adheres to the surface of the image bearing member after the transferring step. The untransferred toner is then removed from the surface of the image bearing member by a cleaning member such as cleaning blade.
With the above configuration, a surface of an image bearing member is sometimes abraded away with time by mechanical stress associated with abrasion by a cleaning member. Such abrasion reduces the operating life of the image bearing member.
Moreover, a surface of an image bearing member is deteriorated by electrostatic discharge caused in between the image bearing member and a charging device when being uniformly charged by the charging device. In recent years, a so-called AC charging device system has become frequently used, in which electrostatic discharge is induced at several hundreds times to several thousands times per second in between a charging member that applies a charge bias containing alternating current components such as a charging roller and an image bearing member. Those AC charging device systems have an advantage in that they produce less acidic gas such as ozone and NOx than electrostatic charger systems, however, they have a disadvantage in that the degree of deterioration of an image bearing member associated with electrostatic discharge is much higher than those of electrostatic charger systems.
As the image quality is improved in recent years, a toner used in image formation tends to be made smaller and more spherical and is likely to pass through between the image bearing member and the cleaning blade. When the toner passes through therebetween, the image quality is degraded due to defective charging of the image bearing member and defective exposure during the optical scanning.
In the meanwhile, an image forming apparatus has been conventionally known, which is equipped with a lubricant coater for applying a zinc stearate powder as a lubricant over a surface of an image bearing member as described in Japanese Patent Application Publication (JP-B) No. 51-22380, for example. In this type of image forming apparatus, the zinc stearate powder applied to the surface of the image bearing member reduces the friction between the image bearing member and the cleaning member, thereby preventing the abrasion of the image bearing member. The zinc stearate powder also reduces the adhesion of residual toner to the image bearing member so that the toner hardly passes through a contact portion between the cleaning member and the image bearing member. Moreover, a film made of the zinc stearate powder absorbs electrostatic discharge energy when being charged to prevent the degradation of the image bearing member due to electrostatic discharge.
However, in this type of the image forming apparatus, the image quality is easily degraded by the generation of fatty acids on the surface of the image bearing member. Specifically, zinc stearate is a metal soap and generates zinc and fatty acids when the zinc stearate is decomposed. When the zinc stearate is decomposed by electrostatic discharge when being charged and fatty acids are generated on the surface of the image bearing member, the lubrication property is degraded, and the toner is likely to adhere to the surface of the image bearing member in a film form. When the adhesion occurs, the resolution of the image is reduced, and the image density becomes uneven. Thus, in a conventional image forming apparatus, a large amount of zinc stearate has been continued to be applied onto an image bearing member to protect the surface of the image bearing member from the fatty acids generated in accordance with the decomposition of the zinc stearate and to maintain excellent lubrication property. Therefore, a mass of the zinc stearates dries up in a short period of time, it is necessary to supply a mass of the zinc stearate many times before the end of the operating life of the image bearing member.
Japanese Patent Application Laid-Open (JP-A) No. 2005-274737 proposes a lubricant mainly composed of higher alcohol having 20 to 70 carbon atom to replace the zinc stearate. According to JP-A No. 2005-27437, in this structure, it is possible to maintain the lubrication property for a long period of time by giving appropriate wettability to the surface of the image bearing member while the higher alcohol stays on the surface of the cleaning member as indefinite particles.
Japanese Patent Application Laid-Open (JP-A) No. 2002-97483 proposes a lubricant composed of a powder of a specific alkylene bis alkyl acid amide compound. According to JP-A 2002-97483, with this lubricant, by placing the lubricant powder on a contact interface between a cleaning member and a surface of an image bearing member, it is possible to maintain the smooth lubrication effects for a long period of time.
However, with the use of the lubricant described in JP-A 2005-274737, a surface of an image bearing member cannot be sufficiently protected from stress caused by electrostatic discharge, although the lubricant can enhance the lubrication property of the image bearing member with respect to a cleaning member and a toner used. Specifically, since the adsorption occupying area per molecule of higher alcohol molecules adsorbed to the surface of the image bearing member can be relatively wide (the molecules can be easily wider on the surface), a sufficient amount of higher alcohol molecules cannot exist in a unit area of the image bearing member. Thus, the stress caused by electrostatic discharge can be easily passed to the surface of an image bearing member via a protective layer made of a lubricant. Thus, the surface of the image bearing member cannot be sufficiently protected from stress caused by electrostatic discharge.
Moreover, the lubricant described in JP-A 2002-97483 is composed of a compound containing nitrogen atoms in its molecules. As the lubricant is decomposed under the stress due to electrostatic discharge, ion dissociatable compounds such as nitrogen oxides and ammonium containing compounds can be generated. When the ion dissociatable compounds are incorporated into a layer of the lubricant, the resistance of the layer of the lubricant is reduced under a high humidity environment. Accordingly, a current is leaked from a latent electrostatic image of an image bearing member, and image blur occurs.